<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<h1 id="h">倒计时</h1>
<input type="text" placeholder="请输入时间">
<button onclick="time()">开始倒计时</button>

<script>
    function time(){
        // 利用id选择器获取元素的方式
        let h1 =document.getElementById("h");
        // 利用选择器获取input元素
        let input = document.querySelector("input");
        // 获取input输入框的值
        let num = input.value;
        let interval = setInterval(function () {
            h1.innerText = num;
            num--;
            if(num<0){
                clearInterval(interval);
                h1.innerText = "时间到";
            }
        },1000);
    }
</script>

</body>
</html>